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Transient numerical calculations were carried out to predict the evolution of temperature and thermal stress in traditionally grown large-size KDP crystals during the removal process, considering two methods that are used to accomplish the crystal extraction. The influence of the crystal size and the difference of temperature between the crystal and environment on the stresses inside the KDP crystals were also investigated in detail. Results indicate that, in both processes of isolating crystals, the highest stress transfers from the crystal periphery to the internal part from the early to the later time stage. In the case of extracting the crystal from solution directly after crystallization and exposing to air, the maximum stress at the crystal periphery is larger than that inside the crystal, and the probability of failure from the outside surface of crystals is large. In the case of retaining the solution for a time after crystallization, the maximum stress in the crystal internal region is larger than that of the crystal surface, leading to a large possibility to originate cracks in the inner region. Both increased crystal sizes and increased temperature differences between the crystals and the environment at the end of crystal growth are factors which aggravate crystal cracking. The maximum stress in crystals in the case of retaining the solution is less than that in the case of extracting the solution, which brings about a decreased likelihood of cracking. Thus, retaining solution for a period of time after the growth is completed, such as 96 h, is suggested to be adopted to accomplish successful crystal extraction.

Potassium dihydrogen phosphate (KDP) crystals were grown from aqueous solutions with different concentrations of cyclohexane diamine tetraacetic acid (CDTA) by the “point seed” rapid growth technique. The effects of CDTA on the surface micro morphology of KDP crystals were systematically studied by atomic force microscopy. The prismatic face growth rates of KDP crystals grown from various solutions were measured by a laser polarization interference system. The results shown that the steps became smooth after CDTA doping. When the doping concentration of CDTA is 100 ppm, the bunching of steps on the (100) face reduced dramatically and the step velocity increased obviously due to the complexation between CDTA and the metal impurities in the original solution. When the doping concentration of CDTA is 500 ppm, the excess CDTA adsorbed on the growth surface and impeded the step movement, leading to an increase in step bunching and reduction in step velocity compared with the situation of 100 ppm.

A series of potassium dihydrogen phosphate (KDP) crystals and Fe3+ doped KDP crystals were grown at different temperatures and at the same supersaturation value of 0.02. The prismatic face growth rates of the KDP crystals were measured using a laser polarization interference system. The surface morphologies of the growth steps on the {100} face were comparatively studied using an atomic force microscope systematically. The results showed that the bunching of steps was related to the supersaturation interval in which it was located, and approached the supersaturation dead zone of σd in which the bunching and the slope of the steps were high. The supersaturation dead zone σd and linear zone σ* decreased with rising temperature. Varying the temperature caused considerable changes in the growth speed, step morphology and step motion speed. For the crystal grown in a doped solution, the bunching and slope of the steps reached a maximum value at around 45 °C. It was interesting that the crystal growth rate in the doped solution was higher than that in the undoped solution at around 65 °C and 75 °C, the bunching of the steps was reduced and the width of the steps widened significantly. At the same supersaturation, the growth rate increased with rising temperature and the steps became straight.

The nonlinear refractive index n2 is an important parameter for the nonlinear optical properties of a medium. This study primarily targets to measure the third-order nonlinear optical properties of potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) with 70% deuteration content at λ = 532 nm using the Z-scan technique. The relationship between nonlinear refraction and direction has been studied in detail. Nonlinear refractive index n2, γ and third-order nonlinear susceptibilities xk3 (esu) were calculated through normalized transmittance curves and theoretical formulas. The results indicate that the nonlinear refractive indexes of KDP and DKDP are positive and possess obvious self-focusing effects in high-power laser systems.

Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, fills up the lacuna left by other two-dimensional materials with a band gap from 0.3 to 2 eV. The narrow direct band gap and the strong light-matter interaction make BP a promising nonlinear optical (NLO) nano-material. In this paper, the NLO properties of multilayer BP nano-sheets were researched by an open aperture z-scan method. Optical limiting (OL) behavior was observed in the BP material. Three different excitation wavelengths (1064, 532 and 355 nm) were used in the experiments, and the long wavelength excitation exhibited superior NLO performance, including OL and saturable absorption (SA). Our results demonstrate that BP is a promising candidate for OL applications.

The band structure of graphene can be adjusted by incorporating other elements or functional groups, correspondingly, the response to an external light field will also be changed. In this study, we investigated the nonlinear optical (NLO) responses of N-doped graphene nanosheets (N-Gns) with an open aperture Z-scan method, under the excitation of picosecond laser pulses. To the best of our knowledge, this is the first time that two-photon absorption (2PA) of N-Gns has been reported. The normalized nonlinear transmission demonstrates that with the increasing excitation energy, the saturable absorption (SA) and the 2PA appear successively. Compared to the 532 nm excitation, the 1064 nm excitation exhibits smaller saturable intensity Is and larger 2PA coefficient β. At the same time, the N-Gns present superior NLO property compared to pure graphene. Consequently, N-Gns will be a promising candidate for SA or optical limiting (OL) applications.